1. Field of Invention
The present invention relates to an electromagnetic wave absorbing material, more specifically, it relates to an electromagnetic wave absorbing material having a high electromagnetic wave absorbing ability in a wide frequency range in the vicinity of 1 to 20 GHz, and the excellent processing property and application property.
2. Related Art
Recently, propagation of mobile communication devices such as portable phones, and radio communication devices such as radio LAN is on the constant rise. According to that, the operation error of the devices, and the problem of the so-called radio interference such as leakage of the information, cross talk, and generation of the noise have been serious. As the countermeasure for the radio interference, an electromagnetic wave absorbing material is used commonly, and various electromagnetic wave absorbing material have been developed and used practically. For example, JP-A-8-172292 discloses an electromagnetic wave absorbing material containing 30 to 60 parts by weight of a furnace black and 5 to 20 part by weight of a carbon short fiber having a 10 to 20 xcexcm fiber diameter and a 0.3 to 1 mm fiber length based on 100 parts by weight of a rubber, particularly effective for an electromagnetic wave of a 1.8 to 2 GHz frequency, which is a PHS (personal handy-phone system) range. Moreover, JP-A-10-27986 discloses an electromagnetic wave absorbing material containing total 80% by weight or less of a carbon black and a carbon fiber having a 0.1 to 50 xcexcm fiber diameter and a 1 xcexcm to 5 mm fiber length, particularly effective for an electric wave in an L band (1 to 2 GHz) and an S band (2 to 4 GHz). These electromagnetic wave absorbing materials are used commonly in a form of a one layer structure with a metal plate or a metal foil jointed on one surface thereof, and are mounted on the wall surface, or the like in the vicinity of an antenna.
In general, an electromagnetic wave absorbing material is produced by forming a resin composition obtained by adding a carbon black or a carbon fiber in a resin and kneading the same into a sheet-like shape. However, since conventional electromagnetic wave absorbing materials including the above-mentioned electromagnetic wave absorbing material contain a large amount of the carbon black or the carbon fiber, a mixing operation is required over a long time in preparation of the resin composition. Moreover, since the resin composition to be obtained is a highly filled material, it has a low flowability, and thus a problem, such as the need of a high shaping pressure, is involved in terms of the processing property. Furthermore, since an electromagnetic wave absorbing material as the shaped product is heavy and has a low flexibility, it is also problematic in terms of the operation property and the application property. In particular, since the used carbon fibers have a large diameter and a long fiber length, the gap in the part wherein carbon fibers are superimposed becomes large with a low filling density when they are dispersed in a resin. Therefore, in order to obtain a sufficient electromagnetic wave absorbing ability, a large amount of the carbon fibers should be contained.
Furthermore, although the electromagnetic wave absorbing ability can be provided with respect to the PHS band, the L band, or the S band, the effect is unknown with respect to a high frequency range higher than, for example, 10 GHz. It is highly expected that the radio communication application range will be spread more and more in the future, and the electric wave to be used will be moved to a higher frequency range accordingly. For example, in the so-called xe2x80x9cintelligent transport system (ITS)xe2x80x9d, a frequency in a several tens of GHz band is scheduled to be allocated. Including the above-mentioned electromagnetic wave absorbing material, electromagnetic wave absorbing materials suitable for the use in such a high frequency range have not been provided so far.
In view of the circumstances, the invention has been achieved, and an object thereof is to provide an electromagnetic wave absorbing material capable of providing the excellent electromagnetic wave absorbing ability in a wide frequency range in the vicinity of 1 to 20 GHz despite a small content of a carbon black, carbon fiber, or the like, and the excellent processing property and application property.
The present inventors found out that an electromagnetic wave absorbing ability same as or higher than that of the conventional embodiment can be obtained even if the amount of the carbon fiber and the carbon black is cut back drastically by using a special carbon fiber produced by the gas phase growth (gas phase growth carbon fiber), and have completed the invention.
That is, in order to achieve the above-mentioned object, the invention provides an electromagnetic wave absorbing material by shaping a resin composition containing 5 to 10 parts by weight of a carbon black and 1 to 10 parts by weight of a gas phase growth carbon fiber in a resin based on 100 parts by weight of the resin.